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China's terracotta army, a collection of 7000 soldier and horse figures in the mausoleum of the country's first emperor, was covered with beaten egg when it was made, scientists say.

According to German and Italian chemists who have analysed samples from several figurines, the egg was as a binder for colourful paints, which went over a layer of lacquer.

"Egg paint is normally very stable, and not soluble in water ... This makes it less sensitive to humidity and moisture," says German co-author Catharina Blaensdorf, a scientist at the Technical University of Munich.

Egg proteins would have also ensured the adhesion of the paint to the lacquer, while also giving the paint thickness and texture, says Blaensdorf's Italian colleague Ilaria Bonaduce, of the University of Pisa.

For the study, which is published in the latest issue of the Journal of Cultural Heritage, the researchers took samples from warrior figurine faces, kneeling archers, swans and paint fragments found on the ground inside the 210 BC mausoleum.

They separated the flakes chemically to isolate the ingredients and then inserted them into a machine to determine their composition.

The researchers thought animal glue might have served as a binder, but all of the data pointed to egg instead.

Bright hues were important "because colour was precious and a colourful army was the best, and an emperor could demand the best", says Blaensdorf.

Built to last

The sturdy terracotta and thick, eggy paint add to the conclusion that the army was also built to last.

The mausoleum was even booby-trapped, with rigged crossbows to stop would-be thieves.

Eighty master potters left their signatures on the terracotta figures. These names show some individuals came from the imperial court, while other artists appear to have been respected local craftsmen.

Some official names overlap with those found on sewage pipes and floor tiles found in other locations.

"So it seems there was an office for making pottery [within] the imperial court," says Blaensdorf.

Historical significance

Erika Ribechini, a scientist in the Department of Chemistry and Industrial Chemistry at the University of Pisa, who did not work on the project, says the new findings "are very well presented".

"Even though the terracotta army is very famous," she says, not much is known about it.

"[The egg discovery] is particularly fascinating in terms of its historical significance, because roughly in the same period, in the Roman Empire and in ancient Greece, the artists used to utilise egg as a binder in creating mural and stone paintings."

The research is likely to help art restorers to repair and preserve the terracotta army.